Chemoreception enables organisms to detect chemicals in its environment. In humans, odor receptor cells are located in the nose. The biochemical receptors for the odorants are transmembrane proteins found in the membrane of receptor cells cilia. Olfactory receptor proteins (ORPs) generally have seven non-intersecting helices.
The detection of environmental chemicals is mediated by peripheral olfactory organs of varied complexity in almost all metazoans. Typically, specialized sensory neurons initiate perception by detecting ambient molecules, commonly called odors, that interact with protein receptors in their membranes. ORPs on the cilia detect the odorants entering the nose. ORPs are coded by approximately 1000 genes, and it is the largest gene family in the genome of any species. ORPs are members of the proteins having seven transmembrane domains, i.e. G-protein couple receptor (GPCR) super-family. They have diverse amino acid sequences and are able to recognize a wide variety of structurally diverse odorants. The amino acid sequences of ORPs are especially variable in the several transmembrane domains. This is a possible mechanism for the recognition of a variety of structurally diverse ligands.
A major path of olfactory transduction is shown in FIG. 1. Binding of the odor molecules to the receptors may activate a G-protein coupled enzymatic cascade to generate second messengers. These messengers can open the ion channels on the membrane of olfactory cells. The opening of channels may depolarize the membrane and lead to action potentials and signaling.